Currently there still exist different opinions on the effect of Sr doping.
Some argued that holes may occupy other Ni 3\(d\)  orbitals and the Hund
coupling may favor a high spin state (\(S=1\) ). This scenario seems
inconsistent with joint analyses of XAS and RIXS experiments [1]} and a number of other calculations [2]}. Nevertheless,
our model allows for a straightforward multi-orbital extension by
considering a two-band Hubbard model of Ni \(3d\)  orbitals (or its projection
at large \(U\) ) plus hybridization with additional conduction bands. So far,
bulk nickelates have not been found superconductive and many issues remain
to be answered both in theory and in experiment [3]}. Our proposal
of the self-doping effect, the Mott-Kondo scenario, and the \(t\) -\(J\) -\(K\) 
model provides a promising starting basis for future investigations.
